In addition to high speeds, high precision, and high efficiency, recent years have seen an orientation in the field of cutting toward zero-emission cutting and dry cutting. In addition, with progress in industrial technology, there has been growing activity in industries that use new materials and materials that are difficult to cut such as those used in aircraft, space development, nuclear power generation, and the like. It is expected that qualitative diversity and quantitative expansion will continue to take place, clearly requiring cutting technology to adjust to these developments.
In particular, the temperature during cutting of the cutting edge of the tool tends to increase under these conditions, leading to reduced tool life. To overcome this problem, cutting tools need to be provided with improved wear resistance, oxidation resistance, and the like.
In response, various types of coated cutting tools have been proposed and implemented. For example, in one known cutting tool, wear resistance and surface protection is improved by coating the surface of a cutting tool formed from a WC-based cemented carbide, cermet, high-speed steel, or the like or a hard substrate of a wear-resistant tool or the like. For the coating, an AlTiSi-based film is used as a hard coating layer (e.g., (AlxTi1-x-ySiy)(NzC1-z), where 0.05≦x≦0.75, 0.01≦y≦0.1, and 0.6≦z≦1). (Japanese Patent Publication Number 2793773, (Japanese Unexamined Patent Publication Number Hei 07-310174, Patent Document 1)). However, it has not been possible to adequately meet the demands for the advanced characteristics described above with cutting tools of this type.
In another proposed technology, a nitride, carbonitride, oxynitride, or carbo-oxynitride having Ti as its main component and containing an appropriate amount of Si is interleaved with a nitride, carbonitride, oxynitride, or carbo-oxynitride having Ti and Al as its main components, there being at least one layer of each. The layers are disposed so that, in the microstructure of the former, independent phases of Si3N4 and Si are present as independent phases in the nitride, carbonitride, oxynitride, or carbo-oxynitride having Ti as its main component. Thus performance of the cutting tool during dry, high-speed cutting is significantly improved (Japanese Patent Publication Number 3347687 (Japanese Unexamined Patent Publication Number 2000-326108, Patent Document 2)).
According to this proposal, with a conventional TiAlN film, an alumina layer formed through surface oxidation taking place during cutting acts as an oxidation protection film that prevents the inward diffusion of oxygen. However, in dynamic cutting, the outermost alumina layer can easily peel away from the porous Ti oxide layer directly beneath it, resulting in inadequate prevention of oxidation. In contrast, a TiSi-based coating used in this proposal provides extremely good oxidation resistance for the film itself while the formation on the outermost surface of a very fine Ti and Si compound oxide containing Si prevents the formation of the porous Ti oxide layer that was a problem in the conventional technology, thus further improving performance. Furthermore, in this proposed technology, the forming to the TiSi-based coating directly on the TiAl-based film is considered important, and the sequence of coatings is also defined. However, this type of cutting tool is still unable to adequately meet the demand for advanced characteristics described above.
A cutting tool has been proposed with a hard coating for cutting tools having superior wear resistance than conventional TiAlN films. The coating is a hard coating formed from (Alb, [Cr1-αVαc)(C1-dNd) (where 0.5≦b≦0.8, 0.2≦c≦0.5, b+c=1, 0.5≦d≦1, 0.05≦α≦0.95) or from (Ma, Alb, [Cr1-αVα]c)(C1-dNd) (where 0.02≦a≦0.3, 0.5≦b≦0.8, 0.05≦c, a+b+c=1, 0.5≦d≦1, 0≦α≦1, and M is Ti, Nb, W, Ta, or Mo). (Japanese Unexamined Patent Publication Number 2003-034859 (Patent Document 3)).
In this proposal, out of the metal components, Al has a high content, with Cr and V being added. This makes it possible to form cubic AlN, which is metastable phase at standard temperature and pressure, thus providing superior hardness and oxidation resistance. However, when performing high-speed, high-efficiency cutting or dry cutting without any lubricant, these coatings have inadequate hardness and stability at high temperatures, preventing them from adequately meeting the demands for advanced characteristics described above.    [Patent Document 1] Japanese Patent Publication Number 2793773 (Japanese Unexamined Patent Publication Number Hei 07-310174)    [Patent Document 2] Japanese Patent Publication Number 3347687 (Japanese Unexamined Patent Publication Number 2000-326108)    [Patent Document 3] Japanese Unexamined Patent Publication Number 2003-034859